1. Field of the Invention
The present invention generally relates to pyrometallurgical treatment of ores and, more particularly, is concerned with direct reduction of metal oxides. Specifically, the invention relates direct reduction of iron oxides, in a continuous feed, continuous discharge, variable slope, variable diameter rotary kiln.
2. Description of the Prior Art
Attempts to develop a large-scale direct process for manufacturing iron and steel to compete with indirect processes now in use have included trials of virtually every known type of apparatus suitable for the purpose (e.g., pot, reverberatory, regenerative, shaft, rotary, stationary, retort, electric, and various combination furnaces, and fluidized-bed reactors). A variety of reducing agents also have been tried, such as coal, coke, graphite, char, distillation residues, fuel oil, tar, producer gas, coal gas, water gas and hydrogen.
The present invention relates to a rotary-kiln type of direct reduction operation using greenball pellets. Generally, direct processes can be classified on the basis of whether they use solid reductants or gaseous reductants.
A rotary-kiln type of operation for the reduction of iron ore by gaseous reagents has some inherent disadvantages. Operation with the reducing gases under pressure is impractical, for example. Also, because only a small portion of the total volume in a rotary kiln is occupied by reactant solids, the productive capacity per unit of reactor volume is relatively low. These disadvantages may be partly or wholly offset by the ability of a rotary kiln to handle fine materials, operate at high reducing temperatures (1800.degree. to 2000.degree. F.) without sticking of reduced iron powder, and operate in a truly continuous countercurrent manner.
Previous rotary kiln direct reduction processes which use solid carbonaceous materials as the source of reductants avoid the problems associated with gaseous reductants, but typically encounter problems with the efficient utilization of volatile hydrocarbon gas contained in the carbon source. Also, greenball pellets which contain carbon cannot be pre-indurated in a separate facility without burning out the carbon and causing sintering of the pellets. Previous attempts made by various researchers to indurate pellets in a reducing atmosphere (such as in the ACCAR Process and the SLRN Process) have not been successful. In addition, existing direct reduction processes are designed to consume large volumes of high grade raw materials to produce premium quality products, and cannot easily develop dual oxidizing and reducing atmospheres in the same kiln without over-heating the interface area between the two atmospheres or creating the possibility for an explosive condition. Retention time of process material in existing rotary kiln direct reduction processes is on the order of six to eight hours.
Existing rotary kiln direct reduction processes utilize either a countercurrent or co-current gas-to-solids flow system. Countercurrent flow systems (i.e., burden material moves down slope and process gas moves up-slope) cannot efficiently utilize the methane which is evolved from the burden during the preheating period because the temperature in that zone of the kiln is marginal, generally too low for ignition of the gas. Much of the evolved methane passes out of the kiln unburned and must later be burned in the afterburner, which inefficiently wastes the caloric content of the evolved methane.
In co-current flow systems, burden material and process gas flow in the same down-slope direction. A feed-end burner is required to drive the preheating process. Volatile hydrocarbons, which are evolved from the carbon source in the burden during the preheating process, are entrained in other gases and pulled down-slope toward the discharge end of the kiln, in which area the gas is burned with air, the air being introduced by and through auxiliary air blowers. While the energy released by burning the hydrocarbon gases evolved from the burden can be utilized in the reducing process in the co-current system, the exact area or location of the kiln in which the gases burn is very difficult to control and localized overheating can be very detrimental to the process by encouraging ring building on the interior of the kiln refractory. The lack of control is due to the imperfect nature of the burden material and the inexact control of the mechanical feeding equipment which causes the area of release of the volatile hydrocarbon gases to change minute-by-minute in the kiln, thus constant adjustments of the auxiliary air blower dampers are required in order to maintain constant temperature control in the kiln.
Because of the high cost of land, it is also highly desireable to reduce the capital cost of a rotary kiln direct reduction plant by providing a kiln requiring less land without reducing the capacity of the kiln.
Applicants are unaware of any prior art that accomplishes the objects of the present invention. Consequently, a need exists for a method and apparatus for direct reduction of metal oxides and ores by a continuous feed/continuous discharge furnace and a variable slope/variable diameter short rotary kiln.